Vacuum infusion or VARTM (vacuum assisted resin transfer moulding) is one method, which is typically employed for manufacturing composite structures, such as wind turbine blades comprising a fibre-reinforced matrix material. During the manufacturing process, liquid polymer, also called resin, is filled into a mould cavity, in which fibre material priorly has been arranged, and where a vacuum is generated in the mould cavity hereby drawing in the polymer. The polymer can be thermoset plastic or thermoplastics. Typically, uniformly distributed fibres are layered in a first rigid mould part, the fibres being rovings, i.e. bundles of fibres arranged in mats, felt mats made of individual fibres or unidirectional or woven mats, i.e. multi-directional mats made of fibre rovings, etc. A second mould part, which is often made of a resilient vacuum bag, is subsequently placed on top of the fibre material and sealed against the first mould part in order to generate a mould cavity. By generating a vacuum, typically 80 to 95% of the total vacuum, in the mould cavity between the first mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained herein. So-called distribution layers and/or distribution tubes, also called inlet channels, are typically used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases the polymer applied is polyester, epoxy or vinylester, and the fibre-reinforcement is most often based on glass fibres or carbon fibres.
During the process of filling the mould, a vacuum, i.e. an under pressure or negative pressure, is generated via vacuum outlets in the mould cavity, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in all directions in the mould cavity due to the negative pressure as a flow front moves towards the vacuum channels. Thus, it is important to position the inlet channels and vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin.
Resin transfer moulding (RTM) is a manufacturing method, which is similar to VARTM. In RTM the liquid resin is not drawn into the mould cavity due to a vacuum generated in the mould cavity. Instead the liquid resin is forced into the mould cavity via an overpressure at the inlet side.
RTM is less frequently used for or in the manufacture of wind turbine blades than VARTM.
Prepreg moulding is a method in which reinforcement fibres are pre-impregnated with a pre-catalysed resin. The resin is typically solid or near-solid at room temperature. The prepregs are arranged on a mould surface, vacuum bagged and then heated to a temperature, where the resin is allowed to reflow and eventually cured. This method has the main advantage that the resin content in the fibre material is accurately set beforehand. Prepreg moulding may be used in connection with both a RTM and a VARTM process.
Further, it is possible to manufacture hollow mouldings in one piece by use of outer mould parts and a mould core. Such a method is for instance described in EP 1 310 351 and may readily be combined with RTM, VARTM and prepreg moulding.
Blades for wind turbines have increased in size in the course of time and may now be more than 60 meters long and weigh more than 18 tonnes. As a result, the impregnation time in connection with manufacturing such blades has increased, as more fibre material has to be impregnated with polymer. Furthermore, the infusion process has become more complicated, as the impregnation of large shell members, such as blades, requires control of the flow fronts to avoid dry spots.
In order to reduce the weight of wind turbine blades carbon fibres have been used increasingly, as they have a greater strength and rigidity than glass fibres.
Carbon fibres have a considerably smaller diameter than inter alia glass fibres. Consequently, they are compacted to form a very dense structure when subjected to vacuum as in VARTM and also in prepreg moulding. Especially in VARTM, the very dense structure with a very limited amount of voids limits or prevents the propagation and impregnation of the carbon fibres with resin so that dry spots are difficult or impossible to avoid.
WO 2010/018225 discloses a wind turbine blade comprising metal fibres, such as steel fibres. In one embodiment, the wind turbine blade comprises outer layers or skins of glass fibres or carbon fibres in order to obtain a smooth surface.